1. Field of the Invention
This invention relates to an automatic transmission for automobiles with exhaust brake and, more particularly, to a control method of the automatic transmission which automatically causes downshift in response to an exhaust brake control signal to thereby enhance the exhaust brake effect.
2. Description of the Prior Art
In automobiles with automatic transmission, one of the measures for securing the exhaust brake force is to turn on a lock-up clutch, as disclosed in Japanese Patent Application Laid-Open No. 59-80561. However, the conventional measures provide a poor engine brake effect at a high-speed stage or at a low gear ratio. Especially, in the case of an automatic transmission equipped with a one-way clutch, the one-way clutch functions as a free wheel at the time of coasting, hence, the engine brake is scarcely effective and the burden of a foot brake becomes heavy in order to supplement the braking force. Consequently, it will be tried to enhance the effect of the engine brake by turning on an exhaust brake control means and controlling the shift lever of the automatic transmission to achieve downshift, but, such a procedure needs the successive steps of turning on the exhaust brake control means and performing downshift control, as a result, the driver's attention drops, hence, such circumstances are undesirable for safety driving.
That is, the conventional measures need to control the exhaust brake and to perform downshift of the shift lever, thus, such manipulation is troublesome, thereby leaving a question in safety driving.